1. Field of the Invention
The present invention relates to a liquid discharge device that discharges liquid for recording on a recording medium and a cleaning method for a liquid discharge head that discharges liquid.
2. Description of the Related Art
An inkjet head, which is a typical liquid discharge head, includes a plurality of discharge ports from which ink is discharged, flow paths each communicating to a discharge port, and an electrothermal conversion element that generates thermal energy used for discharging ink. The electrothermal conversion element is configured by a heat resistor and an electrode that supplies power to the heat resistor. Covering the electrothermal conversion element with an insulating protection layer having electric insulation ensures insulation between ink and the electrothermal conversion element. The electrothermal conversion element generates thermal energy by being driven. The generated thermal energy quickly heats a contact portion (heat-acting unit) with the ink above the electrothermal conversion element to generate foams, thus causing ink to be discharged.
The heat-acting unit of the inkjet head is heated to a high temperature by the heat of the heat resistor. At the same time, the heat-acting unit is subjected to a physical action, such as an impact by cavitation that occurs as the ink is foamed and foams are shrunk, or to a chemical action caused by the ink. To protect the electrothermal conversion element from these effects, an upper protection layer is provided on the electrothermal conversion element. The upper protection layer is made of metallic materials, such as Ta or the platinum group (Ir, Ru), which are relatively proof against an impact by cavitation and chemical action caused by ink.
At the heat-acting unit, which is a contact portion with the ink, additives included in the ink, such as color materials, are heated to a high temperature and are decomposed and changed into a hard-dissoluble substance. This results in a phenomenon in which the substance is physically adhered to the surface of the upper protection layer that configures the heat-acting unit. This phenomenon is called kogation. As described above, when kogation adheres to the heat-acting unit of the upper protection layer, heat conduction from the heat-acting unit to the ink becomes uneven. This uneven thermal conduction makes foaming unstable, sometimes affecting the discharge characteristics of ink.
Japanese Patent Application Laid-Open No. 2008-105364 discusses a method for solving this issue. According to the method, an upper protection layer is provided that acts as an electrode for generating an electrochemical reaction with the ink and, by an electrochemical reaction, the surface of the upper protection layer is eluted into ink to remove kogation on the heat-acting unit.
FIG. 6A is a schematic diagram illustrating the configuration discussed in Japanese Patent Application Laid-Open No. 2008-105364. An upper protection layer 1107a is formed on an electrothermal conversion element 1117 provided on an inkjet head substrate 1100. This upper protection layer 1107a is exposed in an ink flow path 1110. Voltage is applied in such a way that the upper protection layer 1107a becomes the anode electrode, and an electrode 1107b provided on the same surface on which the upper protection layer 1107a is provided becomes the cathode electrode. Applying the voltage as described above generates an electrochemical reaction between the upper protection layer 1107a and the ink to remove kogation on the upper protection layer 1107a. 